This invention relates to a process for preparing lithium hexafluoroarsenate, which has recently found use as an electrolyte in high-energy density batteries.
U.S. Pat. No. 3,655,332 (see also Honeywell, Inc. Contract NAS 3-10613, Final Report for the period May 31, 1967, to Apr. 30, 1968; Report NASA CR72535, National Aeronautics and Space Administration, Office of Scientific and Technical Information) discloses a process for producing lithium hexafluoroarsenate by a metathetical reaction in an inert organic solvent between lithium tetrafluoroborate and an alkali metal hexafluoroarsenate. The product thus obtained from commercially available lithium tetrafluoroborate is impure and unsatisfactory for use as an electrolyte in long life lithium batteries. This is due to the impurities in commercially available lithium tetrafluoroborate and as yet no satisfactory method for its purification has been developed. Lithium hexafluoroarsenate has also been prepared by (1) neutralization of hexafluoroarsenic acid (HAsF.sub.6) with LiOH, (2) reaction of LiF and AsF.sub.5 in an organic solvent, (3) reaction of LiF, As.sub.2 O.sub.5 and HF in water and fluorination of the LiAsF.sub.5 OH produced with HF, and (4) neutralization of As.sub.2 O.sub.5 with aqueous LiOH, hydrolysis of the LiH.sub.2 AsO.sub.4 produced to LiAsF.sub.5 OH and fluorination of the latter with HF. The products obtained by these methods are contaminated with HF and possibly lithium hydroxyfluoro compounds such as LiAsF.sub.5 OH, which make them unsuitable for use in long life lithium batteries.
Lithium hexafluoroarsenate of high purity suitable for use as electrolyte in long life lithium batteries can be obtained according to the process described in U.S. Pat. No. 3,654,330, which comprises reacting LiAsF.sub.6 with excess CH.sub.3 CN to form Li(CH.sub.3 CN).sub.4 AsF.sub.6, filtering the solution to remove impurities, cooling to precipitate the product and thermally decomposing the product to remove the CH.sub.3 CN and leave a residue of LiAsF.sub.6. However, the process requires LiAsF.sub.6 as starting material and is relatively expensive.